A medium voltage switchgear can involve control units, protections, evaluating and calibration devices. The number of these apparatuses is growing in conjunction with a development of power supplies that brings along a desire for introducing a considerable amount of current or voltage measuring or sensing devices that could be different for every application. It is not possible to place every suitable measuring device in a space available in switchgears or in other applications, not to mention the acquisition costs.
WO 2009/001185 discloses a Rogowski-loop current sensor having a winding extending between two ends and embracing a primary conductor, the current of which is measured. The sensor further includes a closing mechanism provided at the ends of the winding, which ends can be moved away from or towards each other. The closing mechanism includes a body having a high magnetic permeability and extending between the ends when the loop is closed. This solution does not provide measurements using measuring elements with different parameters, ratings or accuracy design inside a switchgear due to the fact that the measuring element consists of just one winding.
WO 2009/139521 discloses a clamp type current sensor with a Rogowski coil, wound on a flexible air-core. The Rogowski coil is accommodated in a circular housing consisting of two semicircular housing members. The housing can be closed by shielding covers engaged with upper open ends of the semicircular housing members. This solution does not provide measurements using measuring elements with different parameters, ratings or accuracy design inside a switchgear due to the fact that the measuring element consists of just one winding.
Current measurements in medium voltage switchgears often can involve different measuring elements for different purposes. Known solutions use two measuring devices when a desire arises to measure, for example, relatively small fluctuations of the current with high accuracy for revenue metering purposes and at the same time to measure high dynamic range or sudden big changes of the current value (e.g., short circuit), for protection and control purposes. The only possibility to conduct both measurements is to change the measuring element. A small space in new medium voltage switchgears may not enable using two separate measuring devices at the same time.
It would be desirable to provide a small and compact measuring device with at least two measuring inputs having different ratings or accuracy of coils used as measuring elements.